Invasion of Siberian Elm (Ulmus Pumila) Along the South Platte River: The Roles of Seed Source, Human Influence, and River Geomorphology

Riparian ecosystems in the western USA have been invaded by non-native woody species deliberately introduced for stream bank stabilization, agricultural windbreaks, and urban shade. Recent work suggests that the non-native tree Ulmus pumila (Siberian elm) is capable of significant spread in western riparian ecosystems, that range infilling is still incomplete, and that the invasion is dispersal-limited. We analyzed the interacting roles of propagule pressure from upland U. pumila, human influence based on road and farmstead density, and river geomorphology in promoting U. pumila invasion into riparian corridors along the South Platte River, Colorado, USA. U. pumila stem density increased with increasing channel and floodplain restriction and increasing human influence from both urban and rural development. Model selection indicated that local upland U. pumila seed sources were relatively unimportant to riparian U. pumila stem density, suggesting that upland propagule pressure is currently contributing less than human influences to U. pumila spread along the South Platte River. In particular, higher road density was the most important predictor for the proportional abundance of smaller U. pumila individuals (DBH < 5-cm and 5-15-cm), suggesting that human influence in densely populated areas has been the primary driver of recent U. pumila population expansion. U. pumila stem density was only weakly associated with abundance of other common riparian tree species. Land managers and other entities concerned with non-native tree invasion into important riparian habitat may be able to reduce U. pumila spread most effectively by focusing U. pumila control efforts where human influences are greatest.

Phenology of Antler Casting and Occurance of Late-Breeding in Nebraska White-Tailed Deer

White-tailed deer (Odocoileus virginianus) in Nebraska primarily breed in November (stimulated by photoperiod), but this season extends into December and January for unfertilized females and healthy fawns reaching the appropriate breeding weight by winter. Variation in the typical mating period can be attributed to geographic region, skewed sex ratios, and fawns reaching sexual maturity during their first year of birth. During April of 2019, we observed late season breeding by a male white-tailed deer in the central Platte River valley and documented late antler casting for this same late-copulating male. An additional male was observed in April of 2020 still retaining antlers within the region. To understand the irregularity of these observations relative to the central Platte River valley, we documented observations of early and late antler casting utilizing long-term cast antler collection and camera trap data. We were able to establish a baseline for antler casting phenology within this region to denote a shift in average casting timeframe (more than one month later than previously recorded in this region) and document early and the latest known antler casting within the literature. Herein, we detail the variation in antler casting phenology for white-tailed deer in this region, and the latest known occurrence of natural breeding in the state. Our findings help us better understand the life history of the Nebraska white-tailed deer population and can assist state wildlife biologists and property managers as they assess seasonal harvest regulations and adaptively manage the changing resource.

Topographic Map Analysis of the North Platte River-South Platte River Drainage Divide Area, Western Larimer County, Colorado, USA Eric Clausen

The United States Supreme Court settled legal disputes concerning four different Larimer County (Colorado) locations where water is moved by gravity across the high elevation North Platte-South Platte River drainage divide, which begins as a triple drainage divide with the Colorado River at Thunder Mountain (on the east-west continental divide and near Colorado River headwaters) and proceeds in roughly a north and northeast direction across deep mountain passes and other low points (divide crossings) first as the Michigan River (in the North Platte watershed)-Cache la Poudre River (in the South Platte watershed) drainage divide and then as the Laramie River (in the North Platte watershed)-Cache la Poudre River drainage divide. The mountain passes and nearby valley and drainage route orientations and other unusual erosional features can be explained if enormous and prolonged volumes of south-oriented water moved along today&rsquo;s north-oriented North Platte and Laramie River alignments into what must have been a rising mountain region to reach south-oriented Colorado River headwaters. Mountain uplift in time forced a flow reversal in the Laramie River valley while flow continued in a south direction along the North Platte River alignment only to be forced to flow around the Medicine Bow Mountains south end and then to flow northward in the Laramie River valley and later to be captured by headward erosion of the east-oriented Cache la Poudre River-Joe Wright Creek valley (aided by a steeper gradient and less resistant bedrock). Continued uplift next reversed flow on the North Platte River alignment to create drainage routes seen today. While explaining Larimer County North Platte-South Platte drainage divide area topographic map drainage system and erosional landform evidence this interpretation requires a completely different Cenozoic history than the geologic history geologists usually describe.

Importance of woodland patches as hibernacula for gartersnakes in a prairie river floodplain of central Nebraska

Gartersnakes are common inhabitants along prairie rivers in the Great Plains, but little information is known about hibernacula among diverse floodplain habitats. We radio-tracked Common Gartersnakes (Thamnophis sirtalis) and Plains Gartersnakes (Thamnophis radix) to hibernacula on islands in a braided river system subject to frequent environmental changes along the Platte River in central Nebraska. We further examined capture rates of gartersnakes in floodplain woodland patches from June to November to examine seasonal use of this habitat. In early and mid-September, movements of snakes with transmitters were in grasslands. From late September to mid-October, the farthest movements were documented, and snakes moved from grasslands into woodland patches. From late October to January, movements were minimal in and around hibernacula in wooded or formerly wooded habitats. Capture rates of gartersnakes in woodland trapping arrays also increased in October and November, further demonstrating woodland use during times when snakes travel to and reside at hibernacula. Although grasslands comprised most of the prairie islands at the study area, observations suggested that the limited woodlands on these islands are important for gartersnakes prior to and during hibernation along the Platte River in central Nebraska. Areas with large trees, such as Plains Cottonwoods (Populus deltoides), appeared to provide overwintering sites. In central Nebraska, riparian woodlands continue to be cleared to enhance habitat for endangered and threatened species such as Whooping Cranes (Grus americana), but some of these islands originally contained trees prior to European settlement. Conservation of at least some woodland habitats appears important for overwintering gartersnakes in central Nebraska.

Groundwater Recharge to Support Wildlife and Water Users

The Heyborne Ponds Recharge Project is a multibenefit project that simultaneously seeks to promote wildlife conservation, to address threatened and endangered species recovery, to support recreation, and to facilitate water availability for agriculture. The project delivers water from the South Platte River to recharge ponds to provide habitat for migrating birds while concurrently providing a mechanism for water to infiltrate into the alluvial aquifer and return to the river at a later time. This temporal shift in the timing of flows in the river provides several benefits. Under Colorado law, groundwater users can pump out of priority only if they augment surface water flows. Further, Colorado has interstate commitments to augmenting downstream flows in the Platte River. Multiple-benefit projects such as Heyborne Ponds represent an untapped opportunity for diverse interests to work together in implementing managed aquifer recharge.